This invention relates generally to photoelectric circuits, and is particularly useful in connection with incremental or absolute digital position measuring systems. In particular, this circuit relates to an improvement in photoelectric circuits of the type in which photosensitive elements are connected to each of the two inputs of a differential amplifier, and in which in each case a resistance is connected in parallel with the photosensitive element.
Photoelectric circuits of this general type have been used, for example, in incremental or absolute digital length or angle measuring systems. In this application the photosensitive elements are used to transduce the light flux modulated by the superimposed divisions of a scale and a scanning plate into electrical signals that are processed to determine the relative position of the scanning plate with respect to the scale.
One such photoelectric circuit is disclosed in the Siemens publication, "Technische Mitteilungen Halbleiter: Neue Fotoelektrische Bauelemente in Anwendingsschaltungen", Order No. B 114/1062/47120. On Pages 11 and 12 of this publication there is shown a known path-voltage transducer, in which to each of the two inputs of a fed-back differential amplifier there is connected a photosensitive element, and in which in each case there is circuited a resistance in parallel to the photosensitive element. This arrangement is well suited for fabrication as a monolithic arrangement of photosensitive elements, since each photosensitive element of the circuit includes a common electrode. One disadvantage of this circuit is its relatively large dark current caused by the bias voltage on the photosensitive elements.